Abstract

Flow patterns generated by medusan swimmers such as
jellyfish are known to differ according the morphology of
the various animal species. Oblate medusae have been
previously observed to generate vortex ring structures
during the propulsive cycle. Owing to the inherent
physical coupling between locomotor and feeding
structures in these animals, the dynamics of vortex ring
formation must be robustly tuned to facilitate effective
functioning of both systems. To understand how this is
achieved, we employed dye visualization techniques on
scyphomedusae (Aurelia aurita) observed swimming in
their natural marine habitat. The flow created during each
propulsive cycle consists of a toroidal starting vortex
formed during the power swimming stroke, followed by a
stopping vortex of opposite rotational sense generated
during the recovery stroke. These two vortices merge in a
laterally oriented vortex superstructure that induces flow
both toward the subumbrellar feeding surfaces and
downstream. The lateral vortex motif discovered here
appears to be critical to the dual function of the medusa
bell as a flow source for feeding and propulsion.
Furthermore, vortices in the animal wake have a greater
volume and closer spacing than predicted by prevailing
models of medusan swimming. These effects are shown to
be advantageous for feeding and swimming performance,
and are an important consequence of vortex interactions
that have been previously neglected.